Nanofibers are being considered for a variety of applications because of their unique properties including high surface area, small fiber diameter, layer thinness, high permeability, and low basis weight. More attention has been focused on functionalized nanofibers having the capability of incorporating active chemistry, especially in biomedical applications such as wound dressing, biosensors and scaffolds for tissue engineering.
Nanofibers may be fabricated by electrostatic spinning (also referred to as electrospinning). The technique of electrospinning of liquids and/or solutions capable of forming fibers, is well known and has been described in a number of patents, such as, for example, U.S. Pat. Nos. 4,043,331 and 5,522,879. The process of electrospinning generally involves the introduction of a liquid into an electric field, so that the liquid is caused to produce fibers. These fibers are generally drawn to a conductor at an attractive electrical potential for collection. During the conversion of the liquid into fibers, the fibers harden and/or dry. This hardening and/or drying may be caused by cooling of the liquid, i.e., where the liquid is normally a solid at room temperature; by evaporation of a solvent, e.g., by dehydration (physically induced hardening); or by a curing mechanism (chemically induced hardening).
The process of electrostatic spinning has typically been directed toward the use of the fibers to create a mat or other non-woven material, as disclosed, for example, in U.S. Pat. No. 4,043,331. Nanofibers ranging from 50 nm to 5 μm in diameter can be electrospun into a nonwoven or an aligned nanofiber mesh. Due to the small fiber diameters, electrospun textiles inherently possess a very high surface area and a small pore size. These properties make electrospun fabrics potential candidates for a number of applications including: membranes, tissue scaffolding, and other biomedical applications. Recently, efforts have focused on using electrospinning techniques to produce nonwoven membranes of nanofibers.
Nanofibers can be used to modify the surface of a substrate. Most nanofiber surfaces have to be engineered to obtain the ability to immobilize biomolecules. Surface modification of synthetic biomaterials, with the intent to improve biocompatibility, has been extensively studied, and many common techniques have been considered for polymer nanofiber modification. For example, Sanders et al. in “Fibro-Porous Meshes Made from Polyurethane Micro-Fibers: Effects of Surface Charge on Tissue Response” Biomaterials 26, 813-818 (2005) introduced different surface charges on electrospun polyurethane (PU) fiber surfaces through plasma-induced surface polymerization of negatively or positively charged monomers. The surface charged PU fiber mesh was implanted in rat subcutaneous dorsum for 5 weeks to evaluate tissue compatibility, and it was found that negatively charged surfaces may facilitate vessel ingrowth into the fibroporous mesh biomaterials. Ma et al. in “Surface Engineering of Electrospun Polyethylene Terephthalate (PET) Nanofibers Towards Development of a New Material for Blood Vessel Engineering” Biomaterials 26, 2527-2536 (2005) conjugated gelatin onto formaldehyde pretreated polyethylene teraphthalate (PET) nanofibers through a grafted polymethacrylic acid spacer and found that the gelatin modification improved the spreading and proliferation of endothelial cells (ECs) on the PET nanofibers, and also preserved the EC's phenotype. Chua et al. in “Stable Immobilization of Rat Hepatocyte Spheroids on Galactosylated Nanofiber Scaffold” Biomaterials 26, 2537-2547 (2005) introduced galactose ligand onto poly(e-caprolactone-co-ethyl ethylene phosphate) (PCLEEP) nanofiber scaffold via covalent conjugation to a poly(acrylic acid) spacer UV-grafted onto the fiber surface. Hepatocyte attachment, ammonia metabolism, albumin secretion and cytochrome P450 enzymatic activity were investigated on the 3-D galactosylated PCLEEP nanofiber scaffold as well as the functional 2-D film substrate.